Devices for optically trapping small particles are better known as “optical traps” or “optical tweezers”. The technique relies on the forces created by one or more laser beams that are refracted by a microscopic object in order to trap, levitate and move that object. By focusing a laser beam though a microscope objective lens down to a very small spot (focal region), particles with high indices of refraction, such as glass, plastic, or oil droplets, are attracted to the intense regions of the beam and can be permanently trapped at the beam's focal region. Biologists are considerably interested in optical traps because minute forces can be measured with sub-picoNewton accuracy on the trapped object. Since such small forces are not accessible by conventional techniques such as scanning-force-microscopy, optical traps have become a major investigation tool in biology.
One (preferred) method to measure such forces includes capturing and analyzing the light after interacting with the particle and computing the change in momentum flux of the light due to interaction with the particle. Capturing all the light exiting the optical trap can be difficult, given that a single-beam trap needs highly marginal rays in order to trap efficiently, but even a high numerical-aperture (NA) lens may not accept all these rays when they have interacted with the particle and are deflected farther off the optic axis. In such a case, it can be difficult to capture and analyze all the light leaving the optical trap. Therefore, to address this issue, dual beam optical traps have been developed. Conventional counter-propagating beam optical traps have been used to trap particles, and measure extremely small transverse forces imparted on those particles. See for example, “Optical tweezers system measuring the change in light momentum flux”, Rev. Sci. Instrum., Vol. 73, No. 6, June 2002. Dual-beam traps are also better than single-beam traps for trapping particles with higher refractive indices.
One drawback to conventional momentum-sensing optical traps, and in particular dual beam counter-propagating optical traps, is that they only detect and measure transverse forces on the particle (x and y directions). Conventional optical traps do not detect and measure axial forces on the particle (z direction). Additionally, counter-propagating optical traps can be difficult to align, that is, to bring the two beam foci together at a common point.
There is a need for an optical trap system (and method) that measures all three components of external forces, and that is easy align.